I. Field of the Invention
This invention relates generally to cardiac electrical stimulation apparatus, and more particularly to apparatus for detecting irregular heartbeat action and for intervening with appropriate stimulation to suit the nature of the particular arrythmia encountered.
II. Discussion of the Prior Art
Many forms of cardiac pacing apparatus are known in the art for treating irregular heartbeat action. These include the relatively simple asynchronous pacemakers as well as various types of more elaborate demand pacemakers. The asynchronous pacemaker merely generates pulses at a fixed rate and is not generally synchronized with the heart's own rate. In the demand pacemaker, the device remains dormant when cardiac activity is normal. However, it includes a means for sensing the absence of a normal beat and, in response, injects artificial stimulating pulses at the appropriate point in the cardiac cycle to thereby maintain near-normal rhythms. Such demand pacers have become very sophisticated with advances in microelectronics which has combined high-complexity circuitry with small physical size and low power consumption. Heart activity can be sensed in both the atrium and ventricles, allowing stimulation to be provided on a demand basis, to either or both the atrium and ventricle and in proper timed relation to mimic the normal cardiac cycle. Such pacemakers are commonly prescribed in treating various types of heart block and bradycardias.
One form of stimulator which has received somewhat less attention is that designed to distinguish between normal high heart rate due to physical or emotional activity (physiologic tachycardia) and abnormal high heart rates not based on physiologic demand (pathologic tachycardia). Pathologic tachycardia results in inefficient pumping of blood by the heart and often may revert to lethal episodes of cardiac fibrillation.
Antitachycardia pacemakers have been used with some success in treatment of supraventricular tachyarrythmias which were found not to respond to drugs. Ventricular tachycardia and fibrillation are far more difficult to diagnose by a pacemaker algorithm, and an error in diagnosis may be, in itself, fatal to the patient. This is especially true in the situation where an incorrect diagnosis is made in connection with a patient in whom an automatic antitachycardia pacemaker or a defibrillator is implanted. A defibrillatory, high-energy discharge or a burst of pacing impulses could be applied to the patient inappropriately. If a defibrillating pulse is applied when not needed, it is wasteful of battery power and highly disconcerting to the patient. If an antitachycardia pulse or pulse pattern is applied and it falls into the vulnerable period, it may, in itself, precipitate ventricular tachycardia or fibrillation.
In prior art systems, the factors used in the algorithm for diagnosis of tachyarrythmias have all been related to the heart's electrical signal. Typically, they may include the R-wave repetition rate, the time interval over which the increase in rate takes place and QRS duration. Each of these factors can be simulated by myopotentials, electromagnetic interference, supraventricular tachyarrythmias and bundle branch block or even physiologic sinus tachycardia with or without aberrant A-V conduction. Thus, a need exists for an antitachyarrythmia pacer which can more accurately distinguish between physiologic and pathologic tachycardias.
The present invention provides a method and apparatus for reliably detecting the onset of pathologic tachycardia and for initiating the operation of a cardioversion pulse generator when an inappropriately high rate is detected. The cardioversion pulse generator then produces a pattern of pulses for capturing the heart and bringing the rate into a safe range appropriate to the metabolic condition of the subject at the time.